1. Field of the Invention
The present invention relates in general to liquid crystal displays. In particular, the present invention relates to preparation methods for preparing alignment surfaces for the liquid crystal material that forms a portion of a liquid crystal display.
2. Description of the Related Art
Liquid crystal display technology is used worldwide in a variety of products ranging from watches through cellular phones to computers. The revenue associated with this industry is estimated to be several billion dollars annually.
An essential requirement, in the manufacture of liquid crystal displays, is the alignment of the liquid crystal molecules by controlling the surfaces within which the liquid crystal layer is sandwiched. The pervasive method for achieving this is to use a polyimide coated surface and to then rub the coated surface with a velvet cloth. This rubbing process realigns the polyimide surface and produces alignment of the liquid crystal molecules in contact with the surface. The two rubbed surface then produces alignment of the liquid crystal cell in a desired way. This rubbing method has been the process of choice for the last three decades of manufacturing displays. However, its been recognized by the industry that a non contact or a non rubbed method of surface alignment is very desirable for future manufacturing, since the rubbing method introduces debris from the cloth in an otherwise clean room environment. The rubbing method can also lead to electrostatic charge build up, which can destroy the transistors below the polyimide surface. Since these transistors are essential for the operation of modern liquid crystal display devices, it is important for the LCD industry to find new methods of providing alignment surfaces that do not risk the integrity of such transistors.
U.S. Pat. No. 5,770,826 to Chaudhari et al. teaches that low energy ion beams can be used to modify the surface of a wide class of materials so that the surface develops a directionality. This directionality, or orientational order, then aligns liquid crystal molecules. It has also been demonstrated that such alignment surfaces formed by non-contact ion beam methods can be used to build liquid crystal display panels. Non-contact ion beam "guns" are available commercially.
A strong driving force in liquid crystal display technology is the cost of manufacturing. One approach to reducing cost, for a given manufacturing technology, is to increase the size of the glass substrate. This enables many displays to be manufactured almost simultaneously, thus reducing the cost per unit.
To achieve the desired uniformity of alignment on the surface of the alignment layer, it is important to have ion beam gun uniformity with respect to the emitted beam, and particularly with respect to beam divergence. The glass substrate, on which the displays are to be built, is rectangular in shape. In the past, beam divergence of the ion beam and the rectangular shape of the glass substrate placed limits on the geometrical shape of the ion beam gun. Specifically, prior art ion beam guns are longer in one direction (e.g., rectangular) and, in order to cover the entire glass substrate, the glass substrate is moved relative to the ion gun to irradiate the full surface of the glass substrate. This is called a scan mode.
As the LCD market develops, it is desirable to manufacture as many L,CD's as possible in the most cost effective manner. In a manufacturing line, this would be achieved by increasing the square surface area of substrate processed during the scan mode. One avenue to increase productivity is to increase the amount of glass substrate irradiated in each sweep of the scan mode. Commercially available ion guns are about 70 cm in length, but it is expected that the next generation of manufacturing lines will need to accommodate substrates having a length over twice that presently used. However, the rate limiting factor to increasing the size/length of substrates has been the inability to uniformly irradiate large surface areas of the substrate. One aspect of the difficulty presented has been maintaining uniformity in the border portion between two irradiated areas processed by either (1) two separate sweeps by the same ion gun or (2) one sweep of two adjacent ion guns.
It is possible to manufacture larger ion guns that can irradiate a larger surface area of the substrate per single sweep. However, developing and manufacturing such "mega-size" ion guns becomes more difficult and expensive as the substrate size increases. In fact, the cost associated with manufacturing large ion beam guns would offset any cost saving achieved by increasing the surface area of substrate irradiated. Thus, the foregoing solution leaves much to be desired.
A need exists for a cost-effective method to irradiate large surface areas of substrate while maintaining uniformity of the alignment surface.